1. Field of the Invention
The present invention relates to a production method of a piezoelectric ceramic suitable to be used for a piezoelectric layer of a variety of piezoelectric elements, for example, a piezoelectric actuator, a piezoelectric buzzer, a sounding body and a sensor, and a production method of a piezoelectric element having a piezoelectric layer composed of the piezoelectric ceramic.
2. Description of the Related Art
As a piezoelectric ceramic mainly containing lead (Pb), zirconium (Zr), titanium (Ti), zinc (Zn) and niobium (Nb), a ternary piezoelectric ceramic expressed by a general formula: Pb(Zn1/3 Nb2/3)O3—PbTiO3—PbZrO3, those obtained by replacing a part of lead atoms by barium (Ba) and strontium (Sr) and those added with a variety of trace of oxides thereto have been conventionally used.
Also, as a piezoelectric ceramic mainly containing lead, zirconium, titanium, zinc, magnesium (Mg) and niobium, a quaternary piezoelectric ceramic expressed by a general formula: Pb (Zn1/3 Nb2/3)O3—Pb (Mg1/3 Nb2/3)O3—PbTiO3—PbZrO3 and those added with a variety of trace of oxides thereto are used.
The piezoelectric ceramics are widely applied to a piezoelectric actuator, a piezoelectric buzzer, a sounding body and a sensor, etc. for being excellent in piezoelectric characteristics, workability and mass productivity as disclosed in the Japanese Examined Patent Publication No. 44-17344, the Japanese Examined Patent Publication No. 45-39977, the Japanese Unexamined Patent Publication No.61-129888, the Japanese Unexamined Patent Publication No. 3-256379, the Japanese Unexamined Patent Publication No.8-151264, the Japanese Unexamined Patent Publication No. 2001-181035, the Japanese Unexamined Patent Publication No. 2001-181036, etc.
In recent years, for devices to which a piezoelectric element is applied, those using a multilayer element wherein piezoelectric layers composed of a piezoelectric ceramic and internal electrode layers are alternately stacked, such as a multilayer piezoelectric actuator, have been actively developed. Such a multilayer actuator has advantages of being compact and able to obtain a large displacement in a small electric field.
However, when producing the multilayer actuator, the normal method is to fire after forming internal electrodes. Therefore, in a conventionally known piezoelectric ceramic fired at 1200° C. or higher, thermoduric expensive precious metals, such as platinum (Pt) and palladium (Pd), have to be used as the internal electrodes and there has been a disadvantage in production costs.
As a more inexpensive internal electrodes material, silver and palladium alloy (Ag—Pd alloy) is known. However, when a content of palladium exceeds 30 wt %, palladium causes a reducing reaction during firing which leads to arising of a crack in a piezoelectric element body or peeling of electrodes and other defaults. Thus, the content of palladium in the alloy is preferably 30 wt % or less. To make the content of palladium in the alloy 30 wt % or less, a firing temperature has to be 1150° C. or lower, preferably 1120° C. or lower based on a silver-palladium system phase diagram. Furthermore, the content of palladium has to be small to reduce the production cost, thus, the firing temperature is required to be as low as possible. For example, to make the content of palladium 20 wt % or less, the firing temperature has to be 1100° C. or lower, preferably 1000° C. or lower.
On the other hand, when the firing temperature is too low, densification for vitrification cannot be attained and a piezoelectric ceramic having a sufficient property cannot be obtained. The conventionally known piezoelectric ceramics of this kind are produced by performing main firing at a high temperature of 1200° C. or so, thus, to attain complete sintering even at a low temperature, troublesome handling was necessary such that starting materials of the piezoelectric ceramic is once subjected to temporary firing, then grinded to obtain powder having a large specific surface area, or pressed during main firing.
Also, to lower the main firing temperature, methods of adding a glass component, such as silicon dioxide (SiO2), or adding excessive lead oxide (PbO) have been proposed. In the method, however, property of the piezoelectric ceramic is deteriorated so it is not preferable.